Problem: $ D = \left[\begin{array}{rr}-2 & 3 \\ 0 & 1\end{array}\right]$ $ E = \left[\begin{array}{rrr}-1 & -1 & 0 \\ 1 & -1 & -2\end{array}\right]$ What is $ D E$ ?
Explanation: Because $ D$ has dimensions $(2\times2)$ and $ E$ has dimensions $(2\times3)$ , the answer matrix will have dimensions $(2\times3)$ $ D E = \left[\begin{array}{rr}{-2} & {3} \\ {0} & {1}\end{array}\right] \left[\begin{array}{rrr}{-1} & \color{#DF0030}{-1} & \color{#9D38BD}{0} \\ {1} & \color{#DF0030}{-1} & \color{#9D38BD}{-2}\end{array}\right] = \left[\begin{array}{rrr}? & ? & ? \\ ? & ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ D$ , with the corresponding elements in column $j$ of the second matrix, $ E$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ D$ with the first element in ${\text{column }1}$ of $ E$ , then multiply the second element in ${\text{row }1}$ of $ D$ with the second element in ${\text{column }1}$ of $ E$ , and so on. Add the products together. $ \left[\begin{array}{rrr}{-2}\cdot{-1}+{3}\cdot{1} & ? & ? \\ ? & ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ D$ with the corresponding elements in ${\text{column }1}$ of $ E$ and add the products together. $ \left[\begin{array}{rrr}{-2}\cdot{-1}+{3}\cdot{1} & ? & ? \\ {0}\cdot{-1}+{1}\cdot{1} & ? & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ D$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ E$ and add the products together. $ \left[\begin{array}{rrr}{-2}\cdot{-1}+{3}\cdot{1} & {-2}\cdot\color{#DF0030}{-1}+{3}\cdot\color{#DF0030}{-1} & ? \\ {0}\cdot{-1}+{1}\cdot{1} & ? & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rrr}{-2}\cdot{-1}+{3}\cdot{1} & {-2}\cdot\color{#DF0030}{-1}+{3}\cdot\color{#DF0030}{-1} & {-2}\cdot\color{#9D38BD}{0}+{3}\cdot\color{#9D38BD}{-2} \\ {0}\cdot{-1}+{1}\cdot{1} & {0}\cdot\color{#DF0030}{-1}+{1}\cdot\color{#DF0030}{-1} & {0}\cdot\color{#9D38BD}{0}+{1}\cdot\color{#9D38BD}{-2}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rrr}5 & -1 & -6 \\ 1 & -1 & -2\end{array}\right] $